Exp 15 - Relative Strength of Some Acids Acids and Bases Acid-Base Indicator Acids taste sour They change litmus from blue to red Examples of acids Hydrochloric acid (HCl) Vinegar or acetic acid (CH3COOH) Orange juice Pepsi or Coke Bases feel slippery on the fingers They turn red litmus blue Examples of bases Sodium hydroxide, NaOH Ammonia (NH3, or NH4OH) Bleach (solution of sodium hypochlorite, NaClO) Acidic Neutral Basic

Exp 15 - Relative Strength of Some Acids Arrhenius definition of acids and bases Acids contain hydrogen HCl, H2SO4, CH3COOH An acid is a species that produces H+ (protons) in water HCl(aq) + H2O  Cl-(aq) + H3O+(aq) Bases contain OH- in their formula KOH, NaOH A base is a species that produces OH- (hydroxyl-) ions when dissolved in water NaOH(s) + H2O  Na+(aq) + OH-(aq)

Exp 15 - Relative Strength of Some Acids Arrhenius definition of acids and bases Aqueous solutions of acids have an increased amount of hydronium ions (H3O+) by donating protons (H+) to H2O HCl(g) + H2O(l)  Cl-(aq) + H3O+(aq) H2SO4(l) + H2O(l)  HSO4-(aq) + H3O+(aq) HSO4-(aq) + H2O(l)  SO42-(aq) + H3O+(aq) Aqueous solutions of bases contain an increased amount of hydroxyl ions (OH-) NaOH(s) + H2O  Na+(aq) + OH-(aq) NH3(g) + H2O(l)  NH4+(aq) + OH-(aq)

Exp 15 - Relative Strength of Some Acids Strong acids (HCl, H2SO4) dissociate completely in water donate all their H+ to H2O to form H3O+ HCl(aq) + H2O  Cl-(aq) + H3O+(aq) They are completely ionized Weak acids (acetic acid, vinegar (CH3COOH)) dissociate partly in water donate a small portion of H+ to H2O to form H3O+ HAc(aq) + H2O Ac-(aq) + H3O+(aq) (acetic acid) (acetate ion) They are partially ionized Solution of 0.1 mol acetic acid in 1 liter of water 0.0004 mol H+ 0.0004 mol acetate ions 0.0996 mol acetic acid

Exp 15 - Relative Strength of Some Acids Strong bases (NaOH, KOH) dissociate completely in water release all their OH- in solution NaOH(s)  Na+(aq) + OH-(aq) They are completely ionized Weak bases (NH3) dissociate partly in water remove a proton (H+) from water, resulting in increase in OH- in aqueous solution NH3(aq) + H2O NH4+(aq) + OH-(aq) They are partially ionized Solution of 0.1 mol ammonia in 1 liter of water 0.001 mol NH4+ 0.001 mol OH- 0.099 mol NH3

Exp 15 - Relative Strength of Acids and Bases Note Metal ions create acidic solutions Al3+ and Zn2+ are Bronsted/Lowry acids Al(H2O)63+(aq) + H2O(l) Al(H2O)5OH2+(aq)+ H3O+(aq) The hydrated Al3+ ion donates a proton to water (Bronsted-Lowry) Al3+ accepts an electron pair from O in H2O (Lewis acid) solution become slightly acidic AlCl3 is the salt of a weak base and a strong acid NH4+ is a Bronsted/Lowry acid NH4+(aq) + H2O(l) NH3 + H3O+(aq) NH4+ donates a proton to water solution becomes slightly acidic NH4NO3 is the salt of a weak base and a strong acid

Dissolving Metal Ions in Water The Acidic Behavior of the Hydrated Al3+ ion Nearby H2O acts as Bronsted-Lowry base: proton acceptor Electron density drawn toward Al3+ Al(H2O)63+ Al(H2O)5OH2+ H3O+ H2O Al(H2O)6 acts as Bronsted-Lowry acid: proton donor

Definition of An Acid Arrhenius acid is a substance that produces H+ (H3O+) in water A Brønsted acid is a proton donor A Lewis acid is a substance that can accept a pair of electrons A Lewis base is a substance that can donate a pair of electrons + OH- • H O H • H+ acid base N H • H N H H + H+ + acid base

Exp 15 - Relative Strength of Some Acids pH and Acidity pH = -log [H3O+] In pure water at 25oC: [H3O+] = 10-7 mol/L [OH-] = 10-7 mol/L pH = 7 [H3O+] = [OH-] pH high  [H3O+] low, [OH-] high pH low  [H3O+] high, [OH-] low Stronger acid has lower pH Weaker acid has higher pH

Exp 15 - Relative Strength of Some Acids Purpose Determine the pH of a series of isomolar (same molarity) solutions, using pH paper, acid-base indicators and a pH meter Determine the relative strength of the acids based on the estimated pH Test the following 0.1 M solutions as described HCl - AlCl3 H3PO4 - Zn(NO3)2 CH3COOH - NH4NO3 NaH2PO4 - vinegar tap water - distilled water

Exp 15 - Relative Strength of Some Acids Experimental (I) Get 4 large test tubes Use a volumetric pipet to put 4.0 mL dH2O water in each tube Mark the meniscus with a Sharpie Pour out H2O and dry tube Determine pH using pH paper and pH indicators Get pH paper (only one kind available) and put them on a piece of paper towel (Use clean dry watch glass instead!!) Get 4 clean and dry test tubes for each solution to be tested! Put 4 mL of the solution to be tested in each of the 4 test tubes

Exp 15 - Relative Strength of Some Acids Experimental (II) Determine pH using pH paper and pH indicators (cont’d) Place 1 drop of each of the solutions on a strip of the wide-range pH paper and record the approximate pH, as compared to the color indicators on the pH paper container Test the following 0.1 M solutions and tap water, distilled water HCl - Zn(NO3)2 H3PO4 - NH4NO3 HC3H3O2 - vinegar NaH2PO4 Al(NO3)3

Exp 15 - Relative Strength of Some Acids Experimental (II) Determine pH using pH paper and pH indicators (cont’d) Add 1-2 drops of a different indicator (closest to the pH paper reading) to each of the 10 test tubes that contains one of the following solutions Indicator Choices: thymol blue, methyl orange, methyl red, and bromothymol blue Test the following 0.1 M solutions and tap water, distilled water HCl - Zn(NO3)2 H3PO4 - NH4NO3 HC2H3O2 - vinegar NaH2PO4 - tap water Al(NO3)3 -distilled water Estimate pH of the solutions by comparing the results with figure D.1 or the next 2 slides Estimate the pH as accurate as possible (up to 0.1-0.5 pH units) Record your results Repeat for each of the solutions that you will test

Exp 15 - Relative Strength of Some Acids Acid-Base Indicators pH HInd H+ + Ind-

Exp 15 - Relative Strength of Some Acids Acid-Base Indicators Color 1 Color 2 Color 3 Thymol Blue pH<1.2: red pH 2.8 - 8.0: yellow pH>9.6: blue Methyl Orange pH< 3.1: red pH> 4.5: yellow Methyl Red pH<4.2: red pH>6.3: yellow Bromothymol Blue pH<6.0: yellow pH>7.6: blue Phenolphthalein pH<8.0: colorless pH>: pink/red

pH Indicators Methyl Red Thymol Blue Methyl Orange Bromothymol Blue Phenolphthalein

Exp 15 - Relative Strength of Some Acids Experimental (III) Measuring pH with a pH meter Calibrate pH meter with solutions of known pH according to the instructions (will be given by instructor) Measure and record the pH of every 0.1 M solution to the nearest 0.01 pH unit Record the data and write the data on the board

This Thursday Exp 15: Post-lab Exp 16A: Equilibria with Weak Acids and Weak Bases Prelab preparations Prelab exercises